The present invention relates to a method and an apparatus for manufacturing low temperature fired ceramics which are especially useful in electronic components or parts and further, useful in other various applications, such as heat-resistant industrial articles, tablewares, kitchen utensils and decorative articles.
Recently, there has been growing demand for a more densely integrated electronic circuits with the increasing trend toward miniaturization of computers or electronic devices of various equipment or facilities for public-utility. In such circumstances, substrates must satisfy the following requirements.
(1) Low cost PA0 (2) Light weight PA0 (3) High mechanical strength PA0 (4) The substrates have a high thermal conductivity, thereby allowing rapid radiation of generated heat. PA0 (5) In order to increase the density of a two-dimensional wiring a conductor width of 150 .mu.m or finer can be achieved. PA0 (6) The substrate sheets can be readily stacked in a multilayer configuration and thereby permit a three-dimensional wiring with an increased density. PA0 (7) Passive parts, such as inductors, resistors or capacitors, can be integrally incorporated into a multilayered substrate in the case where a substrate sheet is multilayered. PA0 (8) In order to minimize an electrostatic capacity between conductors, thereby realize rapid response to signals, their dielectric constant must be as small as possible. PA0 (9) For the requirement (8), conductive materials with a low electrical resistance, for example, Ag, Ag-Pd, Cu, Au, can be employed. PA0 (10) Dielectric layers have a small coefficient of thermal expansion near that of a Si semiconductor, i.e., 3.5.times.10.sup.-6 /.degree.C., to be packaged thereon, thereby permitting the direct bonding of semiconductor chips thereto. PA0 (11) Electrical resistivity between wires printed on a substrate is high. PA0 (12) They are not adversely affected by temperature, humidity or other atmospheric conditions and have high stability properties. PA0 (1) The substrates are subjected to warpage and cracking during repeated soldering or dip brazing or other similar thermal operations because of their poor heat resistance, low strength and unfavorably excess thermal expansion coefficient of the order of 50.times.10.sup.-6 /.degree.C. and further their electrical resistivity will be detrimentally affected due to elevated temperature exposure. PA0 (2) When elements, such as resistors or semiconductor IC chips, which tend to generate heat, are packaged on the conventional organic resin substrates, various special considerations or designs, for example, an enlarged bonding area for these elements or the use of radiating sheets, are required in order to prevent the elements from being heated up beyond acceptable temperature levels. PA0 (3) In the fabrication of multilayer structures, it is very difficult to form fine conductors with a width of 150 .mu.m or finer or numbers of through holes with a diameter of smaller than 200 .mu.m. PA0 (4) Reliability in heat or humidity stability is low. PA0 (1) They are manufactured at a very high temperature of 1600.degree. to 1700.degree. C. in an atmosphere of hydrogen and such special conditions considerably increase their production cost compared with the former organic resin multilayer substrates. PA0 (2) Since W or Mo is employed the conductive material, the resistance of the conductors is high. PA0 (3) They exhibit a high density (from 3.8 to 3.9 g/cm.sup.3) as compared to that of the organic resin substrate (2.0 g/cm.sup.3). PA0 (4) Their dielectric constant (9 to 10 at 1 MHz) is high as compared to the dielectric constant of the former organic substrates (3 to 5 at 1 MHz). PA0 (5) Although they exhibit an appreciably lowered thermal expansion compared with the former organic substrate, their thermal expansion coefficient of 7.0.times.10.sup.-6 /.degree.C. (from room temperature to 250.degree. C.) is too large as compared to that of Si semiconductor. PA0 (a) 50 to 65% of powder glass consisting of 10 to 55% of CaO, 45 to 70% of SiO.sub.2, 0 to 30% of Al.sub.2 O.sub.3 and up to 10% of impurities; and PA0 (b) 50 to 35% of powder Al.sub.2 O.sub.3 containing up to 10% of impurities.
Heretofore, for the above mentioned requirements, various organic multilayer substrates or alumina ceramic multilayer substrates have been employed. However, these conventional substrates are superior in some properties, but, in other properties, they are inferior. Therefore, any substrate heretofore available can not give satisfaction in all respects and presents problems or difficulties in practical applications.
More specifically, in the case of organic multilayer substrates, circuits are formed on copper cladding layers bonded onto the both sides of phenol resin or epoxy resin substrate sheets, then the resin sheets are bonded to form a multilayer configuration by using epoxy resin adhesives and through holes are formed between the conductors for interconnection by a mechanical perforation process. However, the organic resin multilayer substrates thus fabricated have the following problems:
On the other hand, multilayered alumina substrates have the following disadvantages:
In view of the above circumstances, there have been proposed several low temperature fired glass ceramic compositions.
As examples of low temperature fired ceramics, low temperature fired ceramics using crystallized glass are presented in some reports. For example, as disclosed in Japanese patent application laid-open No. 54-111517, for the purpose of obtaining lower temperature fired ceramics with a high strength, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 base composition containing additives of B.sub.2 O.sub.3 and nucleating agent are fired at a temperature of 900.degree. to 1000.degree. C. to precipitate crystallites of cordierite in the resulting ceramics.
For the same purpose, there have been reported as other examples lower temperature fired ceramics produced from Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 base compositions with additives of B.sub.2 O.sub.3 and nucleating agent. In this composition, in order to increase the strength of the fired products, spodumene is precipitated. In general, in order to make ceramics at lower firing temperatures, the glass component is required to be contained in much larger quantities. However, in that case, it is very difficult to achieve a high strength of 1800 kg/cm.sup.2 or higher. In either the case of MgO-Al.sub.2 O.sub.3 -SiO.sub.2 or Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2, low temperature fired ceramics with a high strength are attained by precipitating high strength crystals therein by heat treating. However, in the cases of these known ceramic compositions, the desired crystals do not precipitate below 900.degree. C. and the compositions are in a glassy phase in the vicinity of 500.degree. to about 800.degree. C. Thus, when fine conductor patterns are printed onto green ceramic sheets and are fired simultaneously together with the ceramics, the printed patterns tend to flow and thereby deform. Therefore, great difficulty has been experienced in the formation of circuits with a high precision. Further, another difficulty has been encountered in the firing process. When green ceramic sheets containing a large amount of an organic binder are fired to remove the organic binder component, it is requested to fire the sheets so as not to cause the organic binder to remain as carbon in the glass phase and, thus, firing should be carried out without softening glass and while maintaining their porosity at the level required to facilitate degassing. For such requirement, a heating rate in the firing step must be about 2.degree. C. per minute and, as shown in FIG. 1, it takes eight hours to heat to 950.degree. C.
Further, there are reported other low temperature fired ceramics using a mixture of a noncrystallized glass and insulating or refractory oxides. The examples are a borosilicate glass (B.sub.2 O.sub.3 -SiO.sub.2) containing additives of refractory materials, such as cyanite or anorthite (Japanese patent application laid-open No. 50-119814) or a mixture of lead borosilicate glass (B.sub.2 O.sub.3 -SiO.sub.2 -Al.sub.2 O.sub.3 -PbO) and insulating oxides, such as forsterite and ZrO.sub.2 (Japanese patent application laid-open No. 58-17695). These ceramics also, as in the case of low temperature fired ceramics using crystallized glasses previously set forth, can not successfully form circuits in a high resolution due to liability to flowing of patterns and, as referred to in Japanese patent application laid-open No. 50-119814, consume a very long firing time of 12 to 18 hours to heat to 900.degree. C. due to a slow heating rate of the order of 1.degree. C. per minute, as shown in FIG. 1. Japanese patent application laid-open No. 58-17695 states that the total firing time can be reduced to 30 to 60 minutes under the process conditions set forth therein, using a furnace used in a thick film formation. However, actually, such a reduced firing time can be applied only to a small quantity of samples. In the case of continuously firing a large quantity of samples, serious problems, such as warpage and cracking, will unavoidably occur because binder components can not be sufficiently removed. Further, in the cases of the noncrystallized glasses, since it is impossible to obtain a required strength level only from them, refractory materials are added to them in order to achieve an acceptable strength level in the composite systems of the noncrystallized glasses and the added refractory materials. However the noncrystallized glass phase does not crystallize even in a final firing stage at 800.degree. to 1000.degree. C. Thus such noncrystallized glass phase softens during firing and causes deformation of patterns. Further, softening of the glass makes degassing very difficult and, thus, the binder component is required to be slowly driven off by firing for a very long time at temperatures below the softening temperature of the glass. FIG. 2(B) shows shrinkage curves during firing for various low temperature fired ceramics. As can be seen from this figure, in both cases of crystallized glass and noncrystallized glass with addition of refractory materials, shrinkage due to softening of glass (noncrystallized or crystallized glass) is observed at temperatures of 200.degree. to 700.degree. C.
As previously described, conventional organic multilayer substrates have many problems in their properties, for example, in reliability, and alumina multilayer substrates are expensive because of very high firing temperature. Further, the alumina substrates are not satisfactory in regard to their properties or performance. Among the low-temperature fired ceramics proposed up to date, some ceramics approach the desired level in their properties. However, peculiar problems associated with the ceramics heretofore available have not yet been solved satisfactorily. For example, they require an unacceptably high production cost and consume a very long time in the firing process involving removing binder components and these disadvantages are not still really improved.